The invention is illustrated in connection with use in a microwave powered electrodeless lamp, and while it can be used with other types of air or gas cooled lamps, the invention finds particular application with electrodeless lamps. Specific examples of microwave powered lamps are disclosed in U.S. Pat. Nos. 3,872,349, 4,042,850, 4,695,757 and 4,485,332 which are incorporated herein by reference. The lamps find application in curing of ink, organic resins and in photolithography.
Briefly, the electrodeless lamps described in the above patents are comprised of a lamp bulb containing a plasma forming medium which is disposed in a microwave enclosure. During operation of the lamp the medium in the bulb is exposed to microwave or other electromagnetic radiation which is coupled to the microwave enclosure, thereby generating a plasma which emits Ultra Violet (UV), visible and infrared radiation. Typically, the microwave enclosure is comprised of a reflector and mesh. The reflector reflects the radiation which is emitted by the bulb out of the enclosure through the mesh, which is operative to contain the microwave energy. The radiation leaving the enclosure is incident upon the material being processed with the UV energy.
The radiation which is emitted by the lamp increases as a function of the input microwave energy thereby allowing high processing speeds. However, the lamp transfers a great deal of heat to the bulb during operation, and the performance is limited by the effectiveness of bulb cooling techniques. The cooling techniques involve high speed streams of air (in the current designs but other gases could easily be used) impinging on and flowing over the lamp bulb, and carrying heat away as their sensible energy.
It has been found that the cooling air streams, which have to be of high speed to provide adequate cooling for operation of the lamps at high power densities, cause complex transient flow patterns within the reflector cavity and outside it around the material being processed. It has been further discovered that the complex flow patterns include recirculation of air (or other cooling gas) from outside the lamp enclosure into it. This outside gas generally contains products of curing and lithography even in dust-free environment. The high speed jets have been found entrain these contaminants and deposit them on the lamp envelope and reflector surfaces fouling the latter and causing expensive downtime and replacement costs. The prior art solution to this problem has been to provide a quartz shield which reduces the light output and is only partially effective in preventing the reverse flows. Removal of the products of curing and photolithography by an outside flow of air has also been only marginally successful.
It is thus an object of the present invention to provide an improved method and apparatus for preventing the contamination and fouling of lamps which are created by complex recirculating flows caused by streams of cooling air or gas.
It is a further object of the invention to improve the lifetime of the lamp bulbs by eliminating fouling of their surfaces.
It has been found that the reverse flows are drawn into the lamp by regions of vacuum which are caused by entrainment of air or gas by the high speed cooling streams. In accordance with the invention, a source of clean air or gas is provided to satisfy the entrainment requirement of the high speed streams, thus resulting in the elimination of the reverse flows of contaminants.